View Full Version : Another basement question

01-30-2013, 01:34 PM
I am remodeling a basement bathroom/laundry. I am looking for the best waterproofing/insulation/vapor barrier combo. Ive been clued into various potential issues from reading posts here. I have selected RadonSeal to waterproof the block wall where I discovered a small groundwater leak. Now I need to know how this product impacts my insulation decision. I believe the RadonSeal will provide a vapor barrier at the inside face of the block wall. Does that mean I should go with XPS installation to allow inward drying? Or could I use ISO and create another vapor seal? Im constrained to 1.5 furring so I may use 2x2s with 1.5 insulation or 1x3s with insulation in 2 layers. Or would and air space be prefered?

01-31-2013, 11:28 AM
With radon-sealer on the concrete masonry unit (CMU) wall you can put any type of foam you like faced, unfaced EPS/iso/XPS, as long as you don't sandwich any furring between the foam and the wall, and keep the furring on the conditioned-space side. CMU walls do not need a cavity between the foam and the walls to drain, particularly if the interior side of the CMU has been sealed against bulk water and radon/water-vapor. The CMU tolerates water just fine, but has less capillary draw than poured concrete that can sometime threaten the foundation sill of a timber-framed building. The above-grade section would still dry toward the exterior, but there was never ANY drying toward the exterior for the below grade section- the moisture drive is always from the dirt toward the basement, never conversely in a WV location.

If you only have 1.5" to play with between the CMU and gypsum, use a continuous layer of foil-faced iso tacked to the wall with blobs of foam-board construction adhesive, then and tape the seams with FSK tape, then put up vertical 1x2 or 1.3 strapping through-screwed to the CMU 24" o.c. (both fastener spacing and furring spacing= 24") with 2.5" TapCons.

Keep the bottom edges of the iso 1/2-1" off the floor or cut strips of XPS to put under the bottom edges, since iso can slowly wick moisture up from the slab over time. Cut in more 3/4" iso between the furring, to fill up the space, and use can-foam or FrothPack to seal the edges to the furring & floor and to any cut-in insulation you put over the foundation sill & band joist, etc.

Trim any sealing-foam flush, and mount the gypsum to the furring with 1-1/4 wall board screws. As long as you don't use foil or vinyl wallpaper over it the paper facers on the gypsum and the furring will be just fine. If the place floods you'd have to replace the gypsum, but as long as you dried it out within a couple of weeks the furring and foam could stay.

If you use 1x3 strapping 24" o.c. you'd have about an R9-R10 clear-wall value if the CMU is hollow, R8-R9 if they're filled with concrete. While it's cost effective in the longer term to go as high as R15 using EPS (bead-board, like cheap beer coolers), R15 would eat up over 4" of space, not the 2-2.5" we're talking with 1.5" of iso + gypsum.

02-01-2013, 11:28 AM
Thanks Dana! You may have saved me some trouble because I was considering using the sandwich approach. When attaching the ISO to the wall, which direction does the foil side go?

02-04-2013, 12:20 PM
Thanks Dana! You may have saved me some trouble because I was considering using the sandwich approach. When attaching the ISO to the wall, which direction does the foil side go?

Most iso has foil on both sides. I've seen EPS with foil on just one side though...

But in answer to your question, if it's foil on just one side the foil faces the foundation/exterior, and the other side faces the interior side (studwall or furring.)